The present invention relates to the processing of ceramic materials and, in particular, concerns a process for producing improved ceramic materials comprising nitride-containing materials.
Nitride-containing ceramics which are known as xe2x80x98advanced ceramicsxe2x80x99 are used in a wide range of applications, for example in industrial wear parts and bearings, refractories, welding components and molten metal handling materials, cutting tools for metal turning, dies for metal extrusion and wire pulling, military applications and body armour, electronics and composite materials. In aggressive and high temperature environments, the corrosion resistance, strength, toughness and wear resistance of these advanced ceramics offer considerable advantages over the sophisticated metal alloys currently in use.
The process of making metal nitrides by the carbothermal reduction and subsequent or simultaneous nitriding of appropriate metal oxides is well known. A variety of metal or semi-metal nitrides can be made in this way, including silicon nitride, aluminium nitride, boron nitride and titanium nitride. By way of example, an appropriate metal oxide may be mixed with a suitable amount of carbon and the mixture heated to a temperature in the range of from 1300 to 1600xc2x0 C. in a flowing stream of nitrogen gas. Oxygen is removed from the oxide as carbon monoxide, and is replaced by nitrogen, with the result that the oxide is partially or fully converted to the nitride.
Nitride-containing ceramic materials of special interest on account of their superior refractory and mechanical properties are the family of silicon aluminium oxynitride materials, which are known collectively as xe2x80x9csialonsxe2x80x9d. The term xe2x80x9csialonxe2x80x9d which is now widely used to identify this type of material is derived from the chemical symbols Si, Al, O and N of its constituent elements. Sialons are believed to be solid solutions of aluminium oxide in silicon nitride, and those which have the most desirable properties generally have a chemical composition which can be represented by the formula:
Si6xe2x88x92zAlzOzN8xe2x88x92z
where z is greater than zero and less than or equal to 4.2.
Materials of this type are generally prepared by intimately mixing silicon nitride, alumina and aluminium nitride in appropriate proportions, and causing them to react together at a high temperature in an inert atmosphere, which may conveniently be nitrogen gas. Some types of sialon can be made by carbothermal reduction and nitriding of an aluminosilicate. Thus sialon with the chemical composition Si3Al3O3N5 can be made from kaolinite, which is the principal constituent of many clays.
Most natural aluminosilicates, including kaolin clay, usually contain small amounts or iron, either combined with the aluminosilicate, or as the free oxide or oxyhydroxide. It is well known that the carbothermal reduction and nitriding siliceous materials can be catalysed by the presence of iron oxides in the feed material. The conversion of silica to silicon nitride, and the conversion of kaolin to sialon are both catalysed in this way. If insufficient iron is present naturally, extra iron can be added, usually in the form of the oxide. In the presence of free silica, and in the strongly reducing conditions prevailing during the carbothermal reaction, the iron oxide is converted to ferrosilicon. Ferrosilicon is liquid at temperatures above 1250xc2x0 C., and it is generally accepted that the presence of this liquid phase greatly enhances the reduction and nitriding of the remaining silica or aluminosilicate.
However, it has not been widely appreciated that the ferrosilicon can have a deleterious effect on the mechanical and/or refractory properties of the resulting nitride-containing ceramic after sintering.
According to the present invention there is provided a process for treating a nitride-containing ceramic material which includes (1) comminuting the material to produce a particulate ceramic material having a particle size distribution such that at least 40% by weight of the particles have an equivalent spherical diameter (esd) smaller than 2 xcexcm; and (2) applying a ferrosilicon separation step comprising one or both of the following steps:
a) subjecting the particulate ceramic material produced in step 1 to differential sedimentation in a liquid medium to produce substantial separation of a light fraction from a heavy fraction, ferrosilicon required to be separated being included in the heavy fraction, and refined particulate ceramic material being included in the light fraction;
b) subjecting a suspension of the particulate ceramic material to magnetic separation to produce substantial separation of magnetic particulate material from non-magnetic particulate material, refined particulate ceramic material being included in the non-magnetic material.
As described earlier, the present invention involves in the treatment of a nitride-containing ceramic material, eg. a sialon, by a comminution step followed by one or more ferrosilicon removal steps selected from steps a) and b) defined earlier.
Comminution processes are known per se. Comminution of advanced ceramic materials is described for example in Hoyer, J. L., xe2x80x9cTurbomilling: a processing technique for advanced ceramicsxe2x80x9d, Materials and Manufacturing Processes, 9 (4), pages 623-636, 1994.
We prefer to comminute the ceramic material in step (1) by grinding in an aqueous medium using a hard particulate grinding medium.
In step 2 (a) the differential sedimentation is desirably carried out in the presence of a dispersing agent. Preferably, the liquid medium is an aqueous medium.
In step 2 (b) the suspension may be formed in an aqueous medium. The magnetic separation may be effected by application of a magnetic field having an intensity of at least 0.05 tesla in the region of the suspension to be treated.
The nitride-containing ceramic material may be, for example, silicon nitride which is prepared by carbothermal reduction of a silica, or a xcex2xe2x80x2-sialon which is prepared by carbothermal reduction of an aluminosilicate material. In the case of xcex2xe2x80x2-sialon, this may advantageously be prepared in accordance with the method which is described in EP-A-0723932 in which a reaction mixture comprising from 70% to 90% by weight of a hydrous or calcined natural aluminosilicate material, such as a kaolin clay, and from 30% to 10% by weight of a carbonaceous material is calcined at a temperature in the range of from 1300xc2x0 C. to 1600xc2x0 C. in a current of nitrogen gas in an enclosed furnace, wherein the particles of the reaction mixture are maintained in substantially continuous motion relative to one another and to the nitrogen gas and the residence time of the reaction mixture in the furnace is not greater than 3 hours.
Where the comminution step (1) in the method according to the first aspect comprises a wet grinding step such a step may be carried out as follows. The ceramic material is suspended in water to form a suspension containing at least 10% by dry weight of the ceramic material. If the suspension contains more than about 40% by dry weight of the ceramic material, a dispersing agent for the ceramic material is preferably dissolved in the water. The dispersing agent is preferably free of alkali metal cations, since these can cause fluxing of the ceramic material. The dispersing agent may be, for example, ammonia or an ammonium salt of a polycarboxylic acid. A particularly suitable dispersing agent is one which can be substantially completely removed from the ceramic material after the treatment in accordance with the invention. Especially preferred is ammonia solution, which is added in a quantity such as to maintain the pH of the suspension at a value of at least 8.0. The ammonium salt of a polycarboxylic acid, if used, is preferably present in an amount of from 0.1 to 1.0% by weight, based on the weight of dry ceramic material.
The hard particulate grinding medium used in the comminution step (where a wet grinding step) should have a Moh hardness of at least 6, and may comprise, for example, grains of silica sand, alumina, zirconia, or of the product of calcining a kaolinitic clay under conditions such that it is converted predominantly to mullite. The grinding medium preferably consists of particles substantially all of which have a diameter between 100 xcexcm and 5 mm. The grinding medium more preferably has a narrower particle size distribution such that substantially all of the particles have diameters in the range of from 250 xcexcm to 2 mm. Most preferably substantially all of the particles have diameters in the range of from 1 to 2 mm.
A wet grinding step is conveniently performed in a vessel which is provided with an agitator which is rotated by means of an electric motor through suitable transmission means, such as a gearbox or belt or chain drive. Preferably the aqueous suspension of the ceramic material is subjected to agitation with the particulate grinding medium for a time sufficient to dissipate in the suspension at least 300 kJ of energy per dry kilogram of ceramic material.
The differential sedimentation step 2a) may be performed in a centrifuge, or, more preferably, by gravitational sedimentation. The suspension of the comminuted ceramic material is preferably first diluted, if necessary, with water so that the concentration of solids in the suspension is not more than about 20% by dry weight, and a dispersing agent of the type described under step 1) above is added, if it was not already added in step 1). In the process of gravitational sedimentation the aqueous suspension of the ground ceramic material is allowed to stand undisturbed in a suitable container for a time sufficient to permit the desired separation to take place. Different particles in the ground ceramic material settle to the bottom of the container at different rates dependent upon their size and specific gravity. According to Stokes"" Law, the terminal velocity of a particle settling through a fluid under these conditions is given by
v=2r2g(sxe2x88x92r)/9h
where
v is the terminal velocity,
r is the radius of the particle,
s is the specific gravity of the particle,
r is the specific gravity of the fluid,
h is the viscosity of the fluid, and
g is the acceleration due to gravity.
Thus the terminal velocity is a function of the difference in specific gravity between the particle and the fluid, (sxe2x88x92r).
Ferrosilicon has a specific gravity between 5.6 and 6.1, depending on its composition, while nitride-containing ceramics can have a lower specific gravity; for example sialon made from kaolin has a specific gravity of about 3.2. The terminal velocity, and hence settling rate, of a particle of ferrosilicon in water will be over twice as fast as that of a sialon particle of the same diameter. If a suspension of a mixture of nitride-containing ceramic, eg. sialon and ferrosilicon is allowed to sediment, ferrosilicon particles will settle out before nitride-containing, eg. sialon particles of the same size. Thus the differential sedimentation step 2a) serves the dual purposes of removing oversize particles of nitride-containing material, eg. sialon, which would be undesirable in the finished product because they would have a deleterious effect on sintered articles made from the product, and of preferentially removing impurities of high specific gravity, such as ferrosilicon.
Thus, a heavy fraction and a light fraction are obtained by the differential sedimentation. The light fraction, which is separated in a suitable manner eg. by decanting from the heavy fraction, contains the refined ceramic particulate product and the heavy fraction contains the ferrosilicon and other heavy impurities or coarse particles required to be separated. The separate product fraction may be further treated in a known way, eg. by dewatering, eg. by filtration and drying as described below.
The force exerted on a spherical particle of a magnetic material in a magnetic field is given by the formula:   F  =            χ      m        ⁢                  π        ⁢                  xe2x80x83                ⁢                  D          3                    6        ⁢    H    ⁢                  ⅆ        H                    ⅆ        x            
wherein "khgr"m is the volume magnetic susceptibility of the material, D is the diameter of the particle, H is the magnetic field intensity and dH/dx is the rate of change of the magnetic field intensity with distance. From this formula it can be seen that the force on the particle is proportional not only to the magnetic field intensity but also to the rate of change of the magnetic field intensity with distance. Therefore a high-intensity magnetic field which changes rapidly with distance, in other words a very non-homogeneous field, may be used to separate a small particle of magnetic material from a non-magnetic material.
The magnetic separation step 2b) is conveniently performed by passing an aqueous suspension of the comminuted ceramic material through a magnetic separation chamber which is located in a magnetic field of intensity at least 0.05 tesla, and more preferably at least 0.1 tesla. The magnetic field may be provided by permanent magnets which are capable of generating a field of the required intensity, or by means of electromagnet coils. The upper limit of the magnetic field intensity is limited only by the cost of providing and running the apparatus necessary to generate such a field. The magnetic separation may be carried out in a magnetic separation chamber which preferably contains a porous magnetic matrix, or packing, which is of a corrosion resistant, ferromagnetic material, and may comprise, for example, a steel wool, or particles of regular shape, for example spherical, cylindrical or prismatic, or of a more irregular shape, such as filings, cuttings or turnings from a larger piece of a suitable material. The aqueous suspension should contain from 0.1 to 1.0% by weight, based on the dry weight of ground ceramic material, of a dispersing agent of the type described for step 1) above, and preferably has a solids concentration of not more than about 40% by weight, and more preferably not more than 20% by weight, based on the weight of dry ground ceramic material.
By the magnetic separation step, ferrosilicon in the magnetic material is separated from non-magnetic material comprising the refined nitride-containing ceramic material product.
On completion of step 2a) and/or step 2b), the treated ceramic particulate material may be further processed in a known way, eg. it may be dewatered and/or dried using one or more known steps. For example, the material may be dried by spray drying or dewatered, eg. by flocculation, eg. by pH reduction and filtration, followed by thermal drying of the filtered cake.
The dried material may be comminuted, eg. milled, to disperse lumps, packaged as a powder and transported to a user. The powder may include minor amounts of additive, eg. ytrria which will assist subsequent sintering by a user.
The dry powder received by a user may be employed by using ceramic powder processing procedures well known to those skilled in the art. Generally, to preform bodies having a desired shape, the ceramic powder is formed into the shape by castings, moulding, extruding or the like to produce a pre-sintered or xe2x80x98greenxe2x80x99 body followed by one or more sintering steps of the body under appropriate known sintering conditions to give the required shaped ceramic article.
Pressing of a dry powder and slip casting of a wet slurry are two preferred ways of pre-forming the shaped body prior to sintering.
The present invention beneficially allows the known benefits obtained by the use of iron oxide in the feed material employed to produce a nitride-containing ceramic material to be retained whilst minimising or substantially eliminating problems associated with the presence resulting from the use of iron oxide of ferrosilicon in the ceramic product. As noted above, ferrosilicon is liquid at about 1200xc2x0 C. and acts as a fluxing agent. Retention of substantial amounts of ferrosilicon in the ceramic material causes the mechanical properties of bodies sintered using the ceramic material at temperatures above 1200xc2x0 C. to be seriously affected. For example, the mechanical strength and toughness of the body may be seriously reduced and there may be a tendency for the body to deform under stress in a process known as xe2x80x98creepxe2x80x99. However, by minimising the presence of ferrosilicon these problems can be minimised or eliminated, thereby allowing the mechanical properties of the sintered body to be largely unaffected.
The extent to which the ferrosilicon separation steps 2(a) and 2(b) are effective in the removal of ferrosilicon depend upon the amount of ferrosilicon present in the ceramic particulate material and the kind of ferrosilicon present. Usually, ferrosilicon is present mainly as Fe3Si although constituents having compositions ranging from about 90% Si: 10% Fe to about 10% Si: 90% Si may also be present.
As exemplified hereinafter the equivalent Fe2O3 weight content of the particulate ceramic material may be reduced by either step 2(a) or step 2(b) (by a greater extent using both) to less than 10% of its original value (prior to application of the step(s)) thereby substantially minimising the problems caused by residual ferrosilicon.
The residual iron content of the particulate ceramic material expressed as percentage by weight Fe2O3 is desirably less than 0.5, especially less than 0.35, after treatment of the material in accordance with the method according to the first aspect.
According to the present invention in a second aspect there is provided a nitride-containing ceramic particulate material which is the product of the method according to the first aspect. The product may be a powder having a d50 value less than 2 xcexcm, desirably less than 1.5 xcexcm, especially less than 1 xcexcm, wherein d50 is defined as the mean particle size of the particles present in the ceramic particulate material.
Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: